This invention relates to a scanning and/or tracking mechanism intended for use as part of either an optical obstacle avoidance or target tracking airborne radar set. Such radar sets are used on helicopters and other low flying aircraft to detect obstacles or track targets in the aircraft's flight path by transmitting forward of the craft a laser-generated coherent beam in the infra-red region and detecting the echo signals from obstacles such as power lines, hills, trees and buildings.
Such radars require a scanning and tracking mechanism for moving the transmitted beam in some systematic manner over the field of view. For efficient operation, the field of view should extend for approximately 15.degree. to 30.degree. in both azimuth and elevation on all sides of the fore and aft (or roll) axis of the aircraft. Thus a field of view from approximately 30.degree. to 60.degree. is required.
The scanner/tracker which is the subject of this invention is analogous in many ways to the antenna of a microwave radar, and more particularly to a steerable antenna of such a radar. The scanner/tracker utilizes a pair of rotating optical wedges to provide scanning of the radar beam over the field of view. This type of scanning mechanism provides the largest field of view for a given aperture.
The coherent transmitted beam has an extremely narrow beam-width and thus such an optical radar has a capability for extremely high angular (or cross-range) resolution. In order to achieve this high resolution the angular position of the optical wedges which scan the beam must be known with an accuracy comparable to the beamwidth. Thus a high resolution shaft position encoder is required to determine the position of the rotating wedges.
The invention provides such a shaft position encoder. The scanner/tracker is programmable which means that it can be operated in a scanning mode in which the beam will systematically scan the field of view with any one of numerous scanning patterns, or it can track any target of interest. To these ends, the two optical wedges are independently driven by high accuracy positional servo systems which have as one input thereof the output of the system processor. The signal processor processes the received echo signals as well as the output of the aforementioned shaft position encoder to facilitate target tracking.
The scanning system and the servo system are designed for high speed, high accuracy operation and to this end the optical wedges, the moveable encoder disc and the rotor of the servo motor are all integrated into one concentric unit. This structure minimizes inertia and also eliminates the backlash which would result if these units were connected by gears.